Cut-out effect using a QML ShaderEffect

I'd like to achieve a "cutout" effect using a custom QML ShaderEffect item. The area that is cut out should display the pixels of the image (src) but only the pixels that are directly under the ShaderEffect item in the z order. In other words, only the pixels that exist at the same coordinates as the area of cutout square. The final effect would be exactly like if you had two images on top of each other and the top image was being masked in an area to allow the lower image to show through. Like so:

Because of application specific details, I need to achieve this using a custom fragment shader and a pixel shader but am almost a complete stranger to the GLSL language. What I currently have in the code is this:

ShaderEffect {
    id: shader_element
    x: resizeable.x
    y: resizeable.y
    width: resizeable.width
    height: resizeable.height
    property Image src: global_image_reference // from the app's root scope
    vertexShader: "
        uniform highp mat4 qt_Matrix;
        attribute highp vec4 qt_Vertex;
        attribute highp vec2 qt_MultiTexCoord0;
        varying highp vec2 coord;
        void main() {
            coord = qt_MultiTexCoord0;
            gl_Position = qt_Matrix * qt_Vertex;
        }"
    fragmentShader: "
        varying highp vec2 coord;
        uniform sampler2D src;
        uniform lowp float qt_Opacity;
        void main() {
            gl_FragColor = texture2D(src, coord);
        }"
}

I'm passing a global reference to the underlying image (that I want to show through) to the ShaderEffect item and using that reference in the pixel shader. This works but instead of getting a cutout effect, I get a squish effect where the referenced image is being squished when the container is resized:

Any advice on how I need to change either my fragment shader or my pixel shader to achieve the cutout effect instead of the squish effect? I was thinking maybe something utilizing Item's mapToItem() of mapFromItem() functions but I'm not sure how the points returned by those functions can be passed to the vertex or pixel shader.